It started soon after World War II with the introduction of bread and milk and has expanded at such a rapid pace that an American housewife visiting here would have little difficulty shopping in neighborhood shops and finding many familiar labels.

A predominantly fish-eating people have become dedicated meat eaters, if still a long way behind Americans in per capita consumption.

A Health and Welfare Ministry survey just released finds that the average Japanese intake of animal fat and protein last year exceeded vegetables for the first time ever.

But the sudden switch from rice and vegetables to hamburgers and pizzas has its drawbacks: For the first time, some Japanese are beginning to worry about their weight.

"The number of fat Japanese is steadily increasing because of this Wester affluent diet to which we are all exposed," says medical critic Toshiko Shinada. "As a result, a growing number of people are buying low calorie and low fat food products to shed off the excess kilos: Something we Japanese have never thought about before."

The average Japanese daily calorie intake is still only 2,000, compared to the 3,500 calories American daily average.

But for the Japanese government there is a far more worrying aspect to the nation's changing food tastes than merely a few excess inches on the waistline.

The teenager devouring his "quarter pounder" cheeseburger has no idea he is damaging Japan's efforts to achieve greater self-sufficiency in food.

Much of the increased meat consumption, particularly beef, relies on imports.

Japan depends on foreign farmers for almost all its feed grains, 94 percent of its wheat and 95 percent of its soybeans, mostly from the United States.

The only food it has in abundance is rice. Currently almost 7 million tons are going to waste in government warehouses primarily because of a government-subsidized rice production system dating from the Pacific war, which teh authorities dare not scrap in the face of a powerful rural lobby, and a time-honored belief that Japan must keep up its farming capacity. Even taking the rice glut into account, Japan is only 34 percent self-sufficient in grain, a cause of growing concern for government planners.

The Ministry of Agriculture reckons the country would need almost double its present farm acreage to solve the problem --restructuring of the mountainous archipelago or removal of a good part of the population of 118 million from current residential land.

The Japanese have watched how the United States has wielded food as a political weapon (the grain embargo against the Soviet Union over its Afghanistan invasion).

No one in Tokyo seriously believes Japan's main supplier would deliberately try and cut off grain shipments. But there is concern about what would happen in the event of world wide shortages.